Frequency synthesizers can be analog or digital devices. Frequency synthesizers are electronic devices enabling generating a range of frequencies from a single fixed reference oscillator. Frequency synthesizers (also referred to as synthesizers in the following) are commonly found in many modern systems such as communication systems (mobile phones, radio receivers, etc.) or radar systems and different technologies coexist for frequency synthesizer implementation. Traditionally, a common technology for implementing synthesizers is phase locked loop (PLL) modules but recent technical developments have driven migration towards more modern direct digital synthesizer (DDS) modules.
Frequency synthesizers are generally required to provide precise frequencies and therefore a regular check of frequency synthesizers can often be beneficial. Synthesizer testing is aimed at determining whether an actual frequency of the synthesizer output signal coincides with a frequency command (i.e. a requested frequency). Synthesizer testing equipment generally requires access to the input/output of the synthesizer and a frequency analyzer to analyze the output signal.
However, because of the space required for such testing equipment and the need for direct access to the input/output of the synthesizer, such systems remain generally confined to laboratory testing and are not contemplated when the synthesizer is integrated (i.e. enclosed) in a complex electronic system. For example, FIG. 1 shows a surveillance system 500 comprising a radar equipment 300. The radar equipment 300 comprises a frequency synthesizer 100. The surveillance system 500 may comprise a controller 400 (also referred as built-in-test—BIT—controller) configured to carry out test procedures of the radar equipment 300. However, notably because of space limitations for adding a specific frequency test equipment, current radar BIT systems do not include a special test for the frequency synthesizer 100 of the radar 300. Additionally, communication and radar systems commonly function in a frequency range comprised between 1 and 100 GHz and testing equipment is not commonly available for the high frequencies of said frequency range.
Some other synthesizer testing methods involve a power detection unit at the output of the synthesizer. For example, the output of the synthesizer is provided with a coupler for directing a portion of the synthesizer output signal to a power detection unit such as a simple diode. Even though such testing methods present the advantage of being easy to integrate into an electronic system, these methods only enable to detect whether the synthesizer outputs a signal or not, and do not provide precise spectral information regarding the output signal. Therefore, these methods cannot precisely ensure that the frequency output of the synthesizer exactly coincides with a frequency command. Nevertheless, it can be noted that methods involving a power detection unit have been largely implemented for testing analog frequency synthesizers such as PLL modules since these methods enable to effectively detect PLL modules dysfunctions. However, such methods do not provide successful results with more recent DDS direct digital synthesizers.